This invention relates in general to a pressure reducing valve mounted on a piping system for compressed air, etc., to maintain a secondary side fluid pressure at a certain set pressure, and, in particular, to a piston structure for a pressure reducing valve in which the primary side fluid pressure acts on the upper surface of the piston through a pilot valve that is opened and closed by the detection of the secondary side pressure. The piston is operated by a pressure difference between the upper surface and the lower surface of the piston, communicating with the secondary side to operatively open and close a main valve connected to the piston.
This type of pressure reducing valve permits the primary side fluid which is introduced to the upper surface of the piston through a pilot valve, to act on the upper surface of the piston while the fluid escapes through a gap between the piston and a cylinder to regulate the fluid pressure acting on the upper surface of the piston.
As disclosed in Japanese Utility Model Laid-Open No. Sho 45-32717, a piston is provided on its side with two annular grooves in which piston rings are disposed. The piston is inserted into a cylinder so that the piston rings always bulge radially to uniformly contact the inside surface of the cylinder.
The piston becomes inclined by fluid that is jetted from the main valve so that the edge or corner of the piston may rub against the inner peripheral surface of the cylinder. This causes a malfunction whereby the piston no longer slides smoothly in the cylinder. To prevent this malfunction, the two piston grooves are spaced from each other as far as possible so that it becomes difficult to incline the piston. Thus, it is inevitable that the wall thicknesses of the upper annular groove and the upper surface of the piston are thinned.
When the piston is subjected to large vertical vibrations upon the occurrence of chattering in the piston structure, however, the thin wall thickness portion of the outer peripheral edge of the upper surface of the piston is deformed . This deformation is toward the side of the groove and is caused by the severe collision of the upper portion of the piston with the inner wall of a main body when the piston is inclined so that the piston ring is pinched in the groove. As a result, the gap between the piston and the cylinder becomes uneven so that the piston interferes with the cylinder to cause a malfunction (loss of smooth sliding).